Recently, with the progress of internet and the popularization of personal computers, the world-wide data can be exchanged so as to make economical and technical progress. However, most of the internet infrastructures are built by the wires made of different materials, such as optical fibers, cables or CAT-5 patch cores, etc., so that it is very inconvenient to build an internet due to many problems and troubles of such as equipment purchase and piping layout. Moreover, the larger the intranet layout is, the more difficult the intranet maintenance is. Thus, by utilizing WLAN, the aforementioned problems can be resolved, and a humanistic and convenient communication environment can then be provided.
The IEEE 802.11 standard established by IEEE is a WLAN standard, wherein a CCK modulation method is mainly utilized for performing data modulation and demodulation processing.
The CCK modulation method utilized by IEEE 802.11b standard is a M-ary orthogonal modulation, wherein one of M different codewords which are nearly completely orthogonal to each other is selected for use in transmission. For example, when the transmitting rate is 11 Mbps, six bits out of eight-bits data waiting for transmission are utilized to select one CCK codeword from sixty-four CCK codewords (M=64), and then the selected CCK codeword is utilized to modulate this six-bit data, and other two bits of the eight-bits data are modulated by differential quadrature phase shifting key (DQPSK), so that each symbol modulated by codeword can carries eight bits data in transmission. The CCK codewords utilized in IEEE 802.11b are derived according to the formula (1),C=expj(Φ1+Φ2+Φ3+Φ4), expj(Φ1+Φ3+Φ4), expj(Φ1+Φ2+Φ4), −expj(Φ1+Φ4), expj(Φ1+Φ2+Φ3), expj(Φ1+Φ3) −expj(Φ1+Φ2), expjΦ1  (1)wherein C is codeword, and expj(Φ1+Φ2+Φ3+Φ4), expj(Φ1+Φ3+Φ4), expj(Φ1+Φ2+Φ4), −expj(Φ1+Φ4), expj(Φ1+Φ2+Φ3), expj(Φ1+Φ3), −expj(Φ1+Φ2), and expjΦ1 are eight different chips, respectively. In formula (1), Φ1, Φ2, Φ3 and Φ4 are dependent on the eight-bits data waiting for transmission. Assume that d0, d1, d2, d3, d4, d5, d6 and d7 stands for each bit of the eight-bits data respectively, wherein Φ1 is dependent on d0 and d1; Φ2 is dependent on d2 and d3, Φ3 is dependent on d4 and d5 and Φ4 is dependent on d6 and d7. Since there are four possibilities for these phase values Φ1, Φ2, Φ3 and Φ4, such as 0, π/2, π and 3π/2, 44=256 CCK codewords can be generated.
Since CCK codewords have a walsh structure, the demodulator can be implemented by a simple butterfly structure. Please referring to FIG. 1, FIG. 1 is a diagram showing a conventional basic fast walsh block demodulation device. An input signal 202 of conventional basic fast walsh block demodulation device 200 is soft decision data of eight chips, such as x0, x1, x2, −x3, x4, x5, −x6 and x7 shown in FIG. 1 Sixteen correlative results regarding x0, x1, x2, −x3, x4, x5, −x6 and x7 can be resolved and obtained through the processes of first level 204, second level 206 and third-level 208 in the conventional basic fast walsh block demodulation device 200.
Please referring to FIG. 2, FIG. 2 is a diagram showing a conventional CCK demodulation system utilizing the conventional basic fast walsh block demodulation devices. As shown in FIG. 2, the conventional demodulation system 250 needs to use four conventional fast walsh block demodulation devices 200 for outputting sixty-four correlative results regarding x0, x1, x2, −x3, x4, x5, −x6 and x7, and then to use a pyramid-like arrays (not shown) composed of many comparators to compare the outputs from four conventional basic fast walsh block demodulation devices 200 for obtaining an output result which has the most correlation.
Moreover, as shown in FIG. 1, in the conventional basic fast walsh block demodulation device 200, a circuit structure of the first level 204 needs to use four adders (operators), and a circuit structure of the second level 206 needs eight adders (operators), and a circuit structure of the third-level 208 needs sixteen adders (operators), so that the conventional basic fast walsh block demodulation device 200 is constructed by at least twenty-eight adders totally. Because a conventional CCK demodulation system needs to use four conventional basic fast walsh block demodulation devices 200, therefore at least 112 adders (operators) in total need implementing. Furthermore, in order to pick an output result having the most correlation from the outputs of four conventional basic fast walsh block demodulation devices 200, many comparisons have to be performed on all correlation calculation results, so that at least sixty-three comparators need to be utilized. Therefore, it takes a lot of time to perform the demodulation calculations, thus lowering the working efficiency and meanwhile increasing the cost of hardware fabrication substantially.
On the other hand, referring to table 1, table 1 shows the cross-correlations among the sixty-four CCK codewords. Since those sixty-four CCK codewords utilized in CCK modulation are incompletely orthogonal to each other, such as shown in table 1, the cross-correlations among about 40% CCK codewords are non-zero. This incomplete orthogonal property among CCK codewords lowers the noise and the cross symbol interference resistance of CCK modulation.
TABLE 1CorrelationQuantity814√{square root over (2)}64122√{square root over (2)}8037